In an electrical communications system, it is sometimes advantageous to transmit information signals (e.g., video, audio, data) over a pair of conductors (hereinafter “conductor pair” or “differential pair”) rather than over a single conductor. The conductors may comprise, for example, wires, contacts, wiring board traces, conductive vias, other electrically conductive elements and/or combinations thereof. The signals transmitted on each conductor of the differential pair have equal magnitudes, but opposite phases, and the information signal is embedded as the voltage difference between the signals carried on the two conductors. This transmission technique is generally referred to as “balanced” transmission.
When a signal is transmitted over a conductor, electrical noise from external sources such as lightning, electronic equipment, radio stations, etc. may be picked up by the conductor, degrading the quality of the signal carried by the conductor. With balanced transmission techniques, each conductor in a differential pair often picks up approximately the same amount of noise from these external sources. Because approximately an equal amount of noise is added to the signals carried by both conductors of the differential pair, the information signal is typically not disturbed, as the information signal is extracted by taking the difference of the signals carried on the two conductors of the differential pair, and thus the noise signal may be substantially cancelled out by the subtraction process.
Many communications systems include a plurality of differential pairs. For example, a high speed communications system that is used to connect computers and other devices to local area networks and/or to external networks such as the Internet will typically include four differential pairs. In such a system, channels are formed by cascading plugs, jacks and cable segments. In these channels, when a plug mates with a jack, the proximities and routings of the conductors and contacting structures within the jack and/or plug can produce capacitive and/or inductive couplings. Moreover, in the cable segments of these channels, four differential pairs are usually bundled together within a single cable, and thus additional capacitive and/or inductive coupling may occur between the differential pairs in each cable. These capacitive and inductive couplings give rise to another type of noise that is called “crosstalk.”
“Crosstalk” in a communication system refers to an unwanted signal that appears on the conductors of an “idle” or “victim” differential pair that is induced by a disturbing differential pair. “Crosstalk” includes both near-end crosstalk, or “NEXT”, which is the crosstalk measured at an input location corresponding to a source at the same location (i.e., crosstalk whose induced voltage signal travels in an opposite direction to that of an originating, disturbing signal in a different path), as well as far-end crosstalk, or “FEXT”, which is the crosstalk measured at the output location corresponding to a source at the input location (i.e., crosstalk whose signal travels in the same direction as the disturbing signal in the different path). Both NEXT and FEXT are undesirable signals that interfere with the information signal.
A variety of techniques may be used to reduce crosstalk in communications systems such as, for example, tightly twisting the paired conductors (which are typically insulated copper wires) in a cable, whereby different pairs are twisted at different rates that are not harmonically related, so that each conductor in the cable picks up approximately equal amounts of signal energy from the two conductors of each of the other differential pairs included in the cable. If this condition can be maintained, then the crosstalk noise may be significantly reduced, as the conductors of each differential pair carry equal magnitude, but opposite phase signals such that the crosstalk added by the two conductors of a differential pair onto the other conductors in the cable tends to cancel out.
While such twisting of the conductors and/or various other known techniques may substantially reduce crosstalk in cables, most communications systems include both cables and communications connectors (i.e., jacks and plugs) that interconnect the cables and/or connect the cables to computer hardware. Unfortunately, the jack and plug configurations that were adopted years ago generally did not maintain the conductors of each differential pair a uniform distance from the conductors of the other differential pairs in the connector hardware. Moreover, in order to maintain backward compatibility with connector hardware that is already installed, the connector configurations have, for the most part, not been changed. As such, the conductors of each differential pair tend to induce unequal amounts of crosstalk on each of the other conductor pairs in current and pre-existing connectors. As a result, many current connector designs generally introduce some amount of NEXT and FEXT crosstalk.
Pursuant to certain industry standards (e.g., the TIA/EIA-568-B.2-1 standard approved Jun. 20, 2002 by the Telecommunications Industry Association), each jack, plug and cable segment in a communications system may include a total of eight conductors 1-8 that comprise four differential pairs. By convention, the conductors of each differential pair are often referred to as a “tip” conductor and a “ring” conductor. The industry standards specify that, in at least the connection region where the contacts (blades) of a modular plug mate with the contacts of the modular jack (i.e., the plug-jack mating region), the eight conductors are generally aligned in a row, with the four differential pairs specified as depicted in FIG. 1. As known to those of skill in the art, under the TIA/EIA 568, type B configuration, conductor 5 in FIG. 1 comprises the tip conductor of pair 1, conductor 4 comprises the ring conductor of pair 1, conductor 1 comprises the tip conductor of pair 2, conductor 2 comprises the ring conductor of pair 2, conductor 3 comprises the tip conductor of pair 3, conductor 6 comprises the ring conductor of pair 3, conductor 7 comprises the tip conductor of pair 4, and conductor 8 comprises the ring conductor of pair 4.
As shown in FIG. 1, in the connection region where the contacts (blades) of a modular plug mate with the contacts of the modular jack, the conductors of the differential pairs are not equidistant from the conductors of the other differential pairs. By way of example, conductor 2 (i.e., the ring conductor of pair 2) is closer to conductor 3 (i.e., the tip conductor of pair 3) than is conductor 1 (i.e., the tip conductor of pair 2) to conductor 3. Consequently, differential capacitive and/or inductive coupling occurs between the conductors of pairs 2 and 3 that generate both NEXT and FEXT. Similar differential coupling occurs with respect to the other differential pairs in the modular plug and the modular jack.
U.S. Pat. No. 5,997,358 to Adriaenssens et at. (hereinafter “the '358 patent”) describes multi-stage crosstalk compensating schemes for plug-jack combinations. The entire contents of the '358 patent are hereby incorporated herein by reference as if set forth fully herein. The connectors described in the '358 patent can reduce the “offending” crosstalk that may be induced from the conductors of a first differential pair onto the conductors of a second differential pair in, for example, the plug-jack mating region where the blades of a modular plug mate with the contacts of a modular jack. Pursuant to the teachings of the '358 patent, a “compensating” crosstalk may be deliberately added, usually in the jack, that reduces or substantially cancels the offending crosstalk at the frequencies of interest. The compensating crosstalk can be designed into the lead frame wires of the jack and/or into a printed wiring board that is electrically connected to the lead frame within the jack. As discussed in the '358 patent, two or more stages of crosstalk compensation may be provided, where the magnitude and phase of the compensating crosstalk signal induced by each stage, when combined with the compensating crosstalk signals from the other stages, provide a composite compensating crosstalk signal that substantially cancels the offending crosstalk signal over a frequency range of interest. In two stage compensation schemes, the first stage has a polarity that is opposite the polarity of the offending crosstalk, while the second stage has a polarity that is the same as the polarity of the offending crosstalk (note that either or both of the first and second stages may have multiple sub-stages). The multi-stage (i.e., two or more) compensation schemes disclosed in the '358 patent can be more efficient at reducing the NEXT than schemes in which the compensation is added at a single stage, especially when the second and subsequent stages of compensation include a time delay that is selected and/or controlled to account for differences in phase between the offending signal and the first stage compensating crosstalk signal. Efficiency of crosstalk compensation may be increased if the first stage or a portion of the first stage design is contained in the lead frame wires or is otherwise at little or no delay from the plug-jack mating region.
Although extremely effective, the NEXT compensating scheme of the '358 patent suffers a drawback in that the NEXT margin relative to certain performance standards set forth by industry groups such as the Telecommunications Industry Association (TIA) may tend to deteriorate at low frequencies (e.g., below approximately 100 MHz) when a high crosstalk plug is used with the jack (i.e., a plug having crosstalk levels at the high end of the acceptable range specified in the relevant industry standards), and at high frequency (e.g., beyond approximately 250 MHz) when a low crosstalk plug is used with the jack (i.e., a plug having crosstalk levels at the low end of the acceptable range specified in the relevant industry standards). In particular, when the net compensation crosstalk in a two-stage compensated jack is less than the original crosstalk (i.e. when a high crosstalk plug is inserted into the jack), the plug-jack combination is said to be under-compensated, and the resultant NEXT frequency characteristic builds up to a peak that causes NEXT marginality at low frequencies before a null sets in at a frequency point determined by the inter-stage delays and the magnitudes of the compensating stages.
On the other hand, when the net compensation crosstalk in such a jack is more than the original crosstalk (i.e. when a low crosstalk plug is inserted), the plug-jack combination is said to be over-compensated, and the resultant NEXT frequency characteristic does not have a null, but the slope of the NEXT frequency characteristic gradually increases, tending towards as much as 60 dB/decade at very high frequencies, and thereby exceeding the TIA limit slope of 20 dB/decade.
Thus, while the low frequency performance of a jack can be improved by increasing the “composite” crosstalk compensation level (i.e., the sum of the crosstalk provided by each stage of a multi-stage crosstalk compensation circuit) when a high crosstalk plug is used with the jack, such an action would lead to further deterioration of the high frequency performance of the jack when a low crosstalk plug is used with the jack. Conversely, while the high frequency performance of a jack can be improved by decreasing the composite crosstalk compensation level when a low crosstalk plug is used with the jack, such an action would lead to further deterioration of the low frequency performance of the jack when a high crosstalk plug is used with the jack.
U.S. Pat. No. 7,190,594 to Hashim et al. (“the '594 patent”), which is assigned to the assignee of the present application, discloses communication connectors for simultaneously improving both the high frequency NEXT performance when a low crosstalk plug is used in the connectors and the low frequency NEXT performance when a high crosstalk plug is used. In particular, the '594 patent describes communications connectors which include series inductor-capacitor circuits in which the resultant capacitive coupling is biased so as to reduce the normalized composite crosstalk compensation level with increasing frequency, thereby providing improved high frequency performance without degrading the low frequency performance of the connector.